1. Field of the Invention
The present invention relates to induction cookers. in induction cooking, the substance to be cooked is placed in a magnetic receptacle heated by a flow of currents in its walls caused by a high-frequency electromagnetic field ranging from some kHz to some hundreds of kHz. The high-frequency electromagnetic field is generated by a flat induction coil that is placed on a support placed beneath a glazed ceramic plate on which the receptacle to be heated is placed, this plate being excited by a high-frequency AC electrical current given by an inverted rectifier.
2. Description of the Prior Art
The induction coil is usually formed by a pancake coil made out of a flat and spiral winding of a multiple-strand conductor of copper wires that are insulated from one another by an electrically insulating varnish to optimize the passage of high-frequency current (limitation of the skin effect) and minimize heating losses.
The insulating varnish of such a coil generally does not withstand temperatures of more than 220.degree. C., any more than the material of its support, which is often a molded plastic, withstands such temperatures. This makes it necessary to provide for a cooling of the induction coil through its support by means of the forced-air ventilation of the lower side of the range and also to provide for the thermal insulation of the induction coil with respect to the receptacle forming its load.
Forced-air ventilation increases the cost of an induction cooking range while thermal insulation with respect to the load requires that the induction coil should be moved away from the glazed ceramic plate supporting the receptacle. This increases the gap, reduces the coupling between the induction coil and the receptacle and increases the level of radiated disturbance.
In addition to the cooling, it is sometimes necessary in certain types of cooking, to shield the induction coil against excess heating by reducing the current delivered by the inverted rectifier.
It has also been proposed to make monoconductor induction coils either out of a rigid copper wire with a circular section having a diameter of 1 mm to 1.3 mm positioned in a helical groove hollowed out on the surface of an electrically insulating flat support or out of a copper tape that is wound edgeways and is therefore vertical, in a helix, between two mica plates. This arrangement permits a variation of the pitch of the coil from the center to the periphery in order to homogenize the distribution of temperature at the bottom of the receptacle.
These induction coils, with their supports, have a relatively great thickness. This is a handicap for the compactness of the instruments, especially for the market of domestic cookers.